Coatings for digital detectors

ABSTRACT

Described is a scintillator screen that includes a supporting layer having a phosphor dispersed in a polymeric binder disposed on the supporting layer and a barrier layer disposed on the polymeric binder. The barrier layer includes a non-moisture absorbing polymer selected from the group consisting of polyethylene terephthalate, cellulose diacetate, ethylene vinyl acetate and polyvinyl butyraldehyde. The barrier layer has a thickness of less than 1 micron. An antistatic layer is disposed on the barrier layer. The antistatic layer includes poly(3,4-ethylenedixythiophene)-poly(styrene sulfonate) (PEDOT/PSS) dispersed in a polymer selected from the group consisting of a polyester and a polyurethane. The antistatic layer has a transparency of greater than 95 percent at a wavelength of from about 400 nm to 600 nm.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application No.61/603,980, filed Feb. 28, 2012, entitled FUNCTIONAL COATINGS FORDIGITAL DETECTORS, which is hereby incorporated by reference in itsentirety. This application relates to commonly assigned copendingapplication Ser. No. (Carestream Docket No. 95318 entitled METHOD OFMANUFACTURING DIGITAL DETECTORS); and to commonly assigned copendingapplication Ser. No. (Carestream Docket No. 95318B entitled; ADHESIVELAYER FOR DIGITAL DETECTORS) all filed simultaneously herewith andincorporated by reference herein in their entirety.

FIELD OF INVENTION

The present invention relates to digital radiography screens.

BACKGROUND

Digital radiography is an alternative to film-based imaging technologiesthat rely on photosensitive film layers to capture radiation exposureand thus to produce and store an image of a subject's internal physicalfeatures. With digital radiography, the radiation image exposurescaptured on radiation-sensitive layers are converted, pixel by pixel, toelectronic image data which is then stored in memory circuitry forsubsequent read-out and display on suitable electronic image displaydevices.

The primary objective of a radiographic imaging detector is toaccurately reproduce the organ or the object that is being imaged, whileexposing the patient (in medical and dental applications) or the object(in non-destructive testing) to a minimal amount of x-rays.

In digital radiography, scintillating screens are used to convert x-raysto visible radiation. The visible radiation is converted byphotosensitive elements (e.g., amorphous silicon) into electricalsignals that are processed by associated circuitry. In order toaccurately image an organ or element, and at the same time, minimize theexposure of the patient or the object to the x-rays, it is necessary toplace the scintillator screen in intimate contact with thephotosensitive element, and ensure that the circuitry associated withthe detection of the signal due to the radiation, is able to detect verysmall levels of electrical charge.

For example, creation of scintillator screens by coating a formulationof scintillator particles, polymeric binders, and other additives on apolymeric support is disclosed in U.S. Pat. Nos. 3,883,747 and4,204,125. However, when these scintillator screens and the radiographicdetectors are brought together in intimate contact, the sensitivity ofthe combination has to be manipulated to ensure that electrostaticdischarge (ESD) events generated due to triboelectric phenomena do notinduce artifacts in the radiographic image or damage the radiographicdetector, which require fairly sophisticated & expensive processes tomanufacture.

A number of approaches have been taken to minimize the impact of ESDevents on digital radiographic detectors. The deposition of conductivecoatings on the surface of the detector to spread out the chargegenerated by the ESD event and circuitry to dissipate the charge isdescribed in U.S. Pat. Pub.2010/0091149A1, U.S. Pat. Pub.2008/0237481A1, U.S. Pat. No. 7,532,264 and U.S. Pat. No. 7,902,004.However, this approach leads to the decrease in the overall sensitivityof the photosensitive detector.

It would be desirable to minimize the probability of an ESD event due totribocharging and to diminish the magnitude of the charge generated inthe case of an ESD event, while maintaining the sensitivity of thephotoelectric detector. In addition, it is necessary to mate thescintillator screen and the flat panel detector in a manner that doesnot degrade antistatic protection or hinder detection of the visibleradiation by the flat panel detector.

SUMMARY

According to an embodiment, there is described a scintillator screenthat includes a supporting layer having a phosphor dispersed in apolymeric binder disposed on the supporting layer and a barrier layerdisposed on the polymeric binder. The barrier layer includes anon-moisture absorbing polymer selected from the group consisting ofpolyethylene terephthalate, cellulose diacetate, ethylene vinyl acetateand polyvinyl butyraldehyde. The barrier layer has a thickness of lessthan 1 micron. An antistatic layer is disposed on the barrier layer. Theantistatic layer includes poly (3,4-ethylenedixythiophene)-poly(styrenesulfonate) dispersed in a polymer selected from the group consisting ofa polyester and a polyurethane. The antistatic layer has a transparencyof greater than about 95 percent at a wavelength of from about 400 nm to600 nm.

According to another embodiment there is provided a digital radiographypanel including a scintillator screen having a supporting layer, aphosphor dispersed in a polymeric binder disposed on the supportinglayer, a barrier layer disposed on the polymeric layer and an antistaticlayer disposed on barrier layer. The barrier layer includes anon-moisture absorbing polymer selected from the group consisting ofpolyethylene terephthalate, cellulose diacetate, ethylene vinyl acetateand polyvinyl butyraldehyde. The antistatic layer includes apoly(3,4-ethylenedixythiophene)-poly(styrene sulfonate) dispersed in apolymer selected from the group consisting of a polyester and apolyurethane. The antistatic layer has a transparency of greater than 95percent at a wavelength of from about 400 nm to 600 nm. A flat paneldetector is disposed on the antistatic layer.

There is provided a scintillator screen that includes a supportinglayer, a phosphor dispersed in a polymeric binder disposed on thesupporting layer, a barrier layer disposed on the polymeric bindercomprising a non-moisture absorbing polymer; and an antistatic layerdisposed on the barrier layer. The antistatic layer includespoly(3,4-ethylenedixythiophene)-poly(styrene sulfonate) dispersed in apolymer selected from the group consisting of a polyester and apolyurethane. The antistatic layer has a transparency of greater than 95percent at a wavelength of from about 400 nm to 800 nm. The antistaticlayer has thickness of from about 0.1 μm to about 0.3 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate several embodiments of thepresent teachings and together with the description, serve to explainthe principles of the present teachings.

FIG. 1 is a schematic illustration of a digital radiographic detector.

It should be noted that some details of the FIGS. have been simplifiedand are drawn to facilitate understanding of the embodiments rather thanto maintain strict structural accuracy, detail, and scale.

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to embodiments of the presentteachings, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

In the following description, reference is made to the accompanyingdrawings that form a part thereof, and in which is shown by way ofillustration specific exemplary embodiments in which the presentteachings may be practiced. These embodiments are described insufficient detail to enable those skilled in the art to practice thepresent teachings and it is to be understood that other embodiments maybe utilized and that changes may be made without departing from thescope of the present teachings. The following description is, therefore,merely exemplary.

The issue of minimizing ESD events is important in many areas ofindustrial and consumer importance, including the photography. ESDevents have been recognized as a primary cause for the accumulation ofcharge on film or paper surfaces, which lead to the attraction of dirtand visible physical defects.

It is recognized that that the triboelectric charge can be dissipatedeffectively by incorporating one or more electrically-conductive“antistatic” layers in photographic films. Many approaches of usingantistatic layers on one or to both sides of the support (photographicfilm or paper base) or as subbing layers either beneath or on the sideopposite to the light-sensitive silver halide emulsion layers have beenutilized. In some industries, such as the packaging industry, theantistatic agent is incorporated into the photographic film base itself.U.S. Pat. No. 6,346,370 describes the use of antistatic layers inphotographic films.

A wide variety of electrically-conductive materials, with a wide rangeof conductivities are available for use in these photographic antistaticlayers. The materials are generally categorized as (i) ionic conductorsand (ii) electronic conductors. In ionic conductors (such as simpleinorganic salts, alkali metal salts of surfactants, ionic conductivepolymers, polymeric electrolytes containing alkali metal salts, orcolloidal metal oxide sols) charge is transferred by the bulk diffusionof charged species through an electrolyte. The drawback of these ionicconductors is that their resistivity is dependent on temperature andhumidity, and many of these materials are water-soluble and would leachout of if the moisture level too high, resulting in a loss of antistaticfunction. Also, the conductivity of ionic conductors is not very high,and is suitable for antistatic dissipation applications, where themagnitude of charge that needs to dissipate is not high, and the timeconstant for dissipation does not have to be very short.

The conductivity of antistatic layers of electronic conductors dependson electron mobility (contain conjugated polymers, semiconductive metalhalide salts, semiconductive metal oxide particles, etc.) rather thanionic mobility and is independent of humidity. The conductivity of thesematerials can be very high, and is suitable for antistatic dissipationapplications, where the magnitude of the charge to be dissipated is highand the time constant for dissipation needs to be short. However, thesematerials tend to be difficult to formulate and coat and often impartunfavorable physical characteristics, such as color, increasedbrittleness and poor adhesion, etc.

Within the antistatic patent literature for photographic applications,it is seen that some of the antistatic layers alleviate some problemsbut may aggravate some others. For example, U.S. Pat. No. 3,525,621teaches how to impart antistatic performance to an aqueous coatingcomposition using silica particles, in combination with an alkylarylpolyether sulfonate, but the high solubility of the alkylaryl polyethersulfonate in an aqueous medium causes leaching of other materials in thecoating during processing. U.S. Pat. No. 5,244,728 teaches a binderpolymer including an addition product of alkyl methacrylate, alkalimetal salt and vinyl benzene which, when incorporated in an antistaticlayer for photographic paper, provides solution to the above problem ofbackmark retention, but introduces spliceability issues. U.S. Pat. Nos.5,683,862 and 5,466,536 teach of the use of a mixture of polymers andcopolymers for good printabilty, but this formulation mixturecompromises the stability of the antistatic layer. Further, the adhesionof the antistatic (or any other) layer to a surface is influenced by thesurface characteristics, as well as the formulation, as evidenced inU.S. Pat. No. 4,547,445, which discloses a layer containing gelatin andan inorganic pigment for improved ink-retaining characteristics, andgood adhesion to polyethylene-coated photographic paper, but which doesnot exhibit good adhesion to biaxially oriented polypropylene-coatedphotographic paper. Antistatic protection in photographic applicationsis not transferrable to digital radiography because the problemsassociated with ESD events in digital radiographic detectors are unique,and require inventive solutions, that resolve these problems, withoutintroducing additional problems.

Disclosed herein is a layer structure and a method of manufacturing adigital radiography (DR) panel that minimizes the magnitude of thecharge generated by any stray ESD event. Certain attributes are requiredfor an antistatic layer structure suitable for use in a DR panel. Theantistatic layer structure must be transparent to radiation in the400-600 nm wavelength region of the electromagnetic spectrum. Theantistatic layer is coated on the surface of a scintillator screen. Thesurface resistivity of the antistatic layer must be less than about 10⁵ohms per square. In addition, the scintillator screen and the flat paneldetector must be mated in a manner that does not degrade the antistaticprotection or interfere with the detection of visible light by the flatpanel detector.

FIG. 1 shows a sectional view of component layers of a digitalradiography (DR) panel 10. Scintillator screen 12 has a scintillatorlayer 14 formed on a support 11 that is highly transmissive to incidentx-ray radiation. Flat panel detector (FPD) 20 detects visible radiation.Interposed between the scintillator screen 12 and the FPD 20 is anadhesive layer 17 provided to bind or mate the scintillator screen 12 tothe FPD 20. In embodiments, the scintillator screen 12 includes abarrier layer 18 and an antistatic layer 19. Scintillator layer 14responds to incident x-ray R by emitting photons toward FPD 20.

Support 11 can be made of borosilicate glass, aluminosilicate glass,fusion-formed glass, metal, or plastic, or combinations thereof.

The scintillating screen 12 is responsive to the X-rays passing throughan object to produce light which illuminates the FPD to provide signalsrepresenting an X-ray image. The scintillating screen 12 includes alayer 14 of a prompt emitting phosphor dispersed as a particulate in apolymeric matrix. Suitable prompt emitting phosphors are well known, forexample, rare-earth oxysulfides doped with a rare-earth activator. Thepresent invention preferably uses emitting phosphors such as Gd₂O₂S:Tb,Gd₂O₂S:Eu, Gd₂O₃:Eu, La₂O₂S:Tb, La₂O₂S, Y₂O₂S:Tb, CsI:Tl, CsI:Na,CsBr:Tl, NaI:Tl, CaWO₄, CaWO₄:Tb, BaFBr:Eu, BaFCl:Eu, BaSO₄:Eu, BaSrSO₄,BaPbSO₄, BaAl₁₂O₁₉:Mn, BaMgAl₁₀O₁₇:Eu, Zn₂SiO₄:Mn, (Zn,Cd)S:Ag, LaOBr,LaOBr:Tm, Lu₂O₂S:Eu, Lu₂O₂S:Tb, LuTaO₄, HfO₂:Ti, HfGeO₄:Ti, YTaO₄,YTaO₄:Gd, YTaO₄:Nb, Y₂O₃:Eu, YBO₃:Eu, YBO₃:Tb, and (Y,Gd)BO₃:Eu, orcombinations thereof. In embodiments gadolinium oxy sulfides Gd₂O₂S:Tb,Gd₂O₂S:Eu, Gd₂O₃:Eu are preferred. However, any suitable prompt emittingphosphor material, including doped phosphor materials, can be used inany of the embodiments described herein. A blend of different phosphorscan also be used. The median particle size of the phosphor particle isgenerally between about 0.5 μm and about 40 μm. A median particle sizeof between 1 μm and about 20 μm is preferred for ease of formulation, aswell as optimizing properties, such as speed, sharpness and noise.

The phosphor layer 14 can be prepared using conventional coatingtechniques, where the phosphor powder is mixed with a solution of aresin binder material and coated onto a support 11. The binder can bechosen from a variety of known organic polymers that are transparent toX-rays, stimulating, and emitting light. Polymeric binders commonlyemployed in the art include sodium o-sulfobenzaldehyde acetal ofpoly(vinyl alcohol); chloro-sulfonated poly(ethylene); a mixture ofmacromolecular bisphenol poly(carbonates) and copolymers comprisingbisphenol carbonates and poly(alkylene oxides); aqueous ethanol solublenylons; poly(alkyl acrylates and methacrylates) and copolymers ofpoly(alkyl acrylates and methacrylates with acrylic and methacrylicacid); poly(vinyl butyral); and poly(urethane) elastomers. However, anyconventional ratio phosphor to binder can be employed. Generally,thinner phosphor layers and sharper images are realized when a highweight ratio of phosphor to binder is employed. Phosphor-to-binderratios are in the range of about 7:1 to about 35:1. In embodiments, thephosphor-to-binder ratio is from about 7:1 to about 25:1.

Scintillator screens are prepared by coating a scintillator solution onthe support 11 to form the scintillator layer 14. The scintillatorsolution can be applied using any conventional coating techniques knownin the art. For example, the scintillator solution can be applied ontothe support 11 by spray coating, dip-coating, doctor blade coating, rollcoating, knife coating, or slot die coating. Suitable manufacturingtechniques are described, for example, in U.S. Pat. No. 7,304,317, whichis herein incorporated by reference in its entirety.

In order to provide suitable antistatic protection it is necessary toprovide a barrier layer 18 on the scintillator screen 12. The barrierlayer 18 must not absorb moisture. In addition, the barrier layer cannotbe water soluble. The barrier layer comprises one or more non-moistureabsorbing polymer binders selected from the group consisting ofpolyethylene terephthalate (PET), cellulose diacetate (CDA), ethylenevinyl acetate (EVA) and polyvinyl butyraldehyde (BUTVAR). The barrierlayer has a thickness of from about 1 μm and about 10 μm. The barrierlayer 18 can extend over and be used to seal the edges of the phosphorlayer 14 to the support 11. The barrier layer is disposed directly onthe phosphor layer. The barrier layer 18 is coated from a solventsolution. The barrier layer materials can be applied using the same slotcoating techniques as used with the phosphor layer 14. The barrier layersolution or dispersion can be applied onto the phosphor layer 14 byspray coating, dip-coating, doctor blade coating, roll coating, knifecoating, or slot die coating. The barrier layer has a transparency ofgreater than 95 percent at a wavelength of from about 400 nm to 600 nm.

In embodiments, when polyethylene terephthalate (PET) is used as thebarrier layer, the preferred thickness range is from about 2 μm to about10 μm. In embodiments, when cellulose diacetate (CDA) is used as thebarrier layer, the preferred thickness range is about 1 μm to about 5μm. In embodiments, when ethylene vinyl acetate (EVA) is used as thebarrier layer, the preferred thickness range is about 2 μm to about 10μm. In embodiments, when polyvinyl butyraldehyde (BUTVAR) is used as thebarrier layer, the preferred thickness range is about 2 μm to about 10μm.

The antistatic layer 19 is disposed on the barrier layer 18. Theantistatic layer comprises poly(3,4-ethylenedixythiophene)-poly(styrenesulfonate) (available from Heraeus Company as PEDOT/PSS) dispersed in apolymer selected from the group consisting of a polyester and apolyurethane. Poly(3,4-ethylenedixythiophene)-poly(styrene sulfonate) isa conductive polymer that is transmissive in the wavelength region of400 nm to 600 nm. The antistatic layer has a transparency of greaterthan 95 percent at a wavelength of from about 400 nm to 600 nm. Inembodiments, the transparency is greater than 94 percent at a wavelengthof from about 400 nm to 600 nm. The layer thickness of the driedantistatic layer is from about 0.1 μm to about 0.3 μm. The coverage ofthe poly(3,4-ethylenedixythiophene)-poly(styrene sulfonate) is fromabout 0.1 micrograms/cm² to about 0.5 micrograms/cm². The ratio ofpolymer to poly(3,4-ethylenedixythiophene)-poly(styrene sulfonate) fromabout 0.5 to about 1.5. The surface resistivity is less than 10¹⁰ ohmsper square. This is the maximum value that the surface resistivity canbe and still protect the DR panel 10 against ESD events. In embodiments,the surface resistivity is less than 10⁸ ohms per square, or 10 ⁵ ohmsper square

The antistatic layers can be coated by any suitable method includingspray coating, dip-coating, doctor blade coating, roll coating, knifecoating, slot die coating or spin coating. Spin coating is a procedureused to apply uniform thin films to flat substrates. An excess amount ofa solution is placed on the substrate, which is then rotated at highspeed in order to spread the solution by centrifugal force. Rotation iscontinued while the fluid spins off the edges of the substrate, untilthe desired thickness of the film is achieved. The higher the angularspeed of spinning, the thinner the film. The thickness of the film alsodepends on the concentration of the solution and the solvent.

Drying of any of the layers (phosphor layer 14, barrier layer 18 orantistatic layer 19) described can be achieved by a number of differenttechniques; including, room temperature evaporation, hot plates, ovens,UV, or IR exposure. The drying technique is typically formulationspecific, depending on solvents used and the presence of other addenda.For example: coating solutions that utilize volatile solvents (lowboiling temperature solvents) can be dried through low temperaturesolvent evaporation. While coatings that utilize non-volatile solvents(high boiling temperature solvents) may require the use of elevatedtemperature or any of the other above mentioned techniques to acceleratethe solvent evaporation process.

The Flat Panel Detector (FPD) 20, also referred to as a detector array,may include a PIN diode as photosensor. Multiple photosensors are usedto detect the radiation emitted from the scintillating phosphors andform a photosensor array. The FPD 20 can include various layers such asa p-doped layer, an I-layer (intrinsic or undoped layer), and an n-dopedlayer formed on a metal layer which is itself supported by a substrate,typically glass such as Corning 1737 display glass. A transparent ITO(Indium-Tin Oxide) layer provides conductive traces. A passivation layeradds insulation and surface uniformity. The scintillator screen 12 ismaintained in physical contact between scintillator screen 12 and FPD 20through an optical adhesive. It is important that physical contact bemaintained across the entire active area of the FPD 20, so that uniformand efficient transfer of the converted visible light is achieved. Theglass of the FPD 20 is typically 0.7 mm thick, and susceptible tobreakage, especially for a large-area, such as 43 cm by 43 cm panels. Inembodiments, FPD 20 can include CMOS and CCD as part of the photosensorarray.

Phosphor layer 14 responds to incident x-ray radiation by emittingphotons toward FPD 20. As long as there is good optical coupling betweenscintillator screen 12 and FPD 20, a sufficient amount of the emittedsignal is directed toward FPD 20.

An adhesive layer 17 mates the scintillator screen 12 and FPD 20. Thedimensional requirements of the FPD 20 panel may be as large as 43 cm by43 cm (1865 square centimeters). This requires a uniform adhesive layer17 that contains no gaps, voids or bubbles and has a constant thickness.Where an air gap occurs, the light transmission and the spatialresolution are significantly degraded.

The adhesive layer 17 must be stable and not impact the opticalproperties or antistatic properties of the FPD 20. The adhesive layer 17must provide some dimensional rigidity; however, it must not be so rigidthat it distorts the FPD 20. The adhesive layer 17 must be uniform andwithout voids.

While embodiments have been illustrated with respect to one or moreimplementations, alterations and/or modifications can be made to theillustrated examples without departing from the spirit and scope of theappended claims. In addition, while a particular feature herein may havebeen disclosed with respect to only one of several implementations, suchfeature may be combined with one or more other features of the otherimplementations as may be desired and advantageous for any given orparticular function.

EXAMPLES Scintillator Screens

CSH Scintillator Screen (Representative of Scintillator Screen Surfacewithout an Overcoat that Serves as a Barrier Layer for SubsequentCoatings (Elements 11 and 14 in FIG. 1)):

A drum of SU-21-572 Permuthane dispersed in solvent from Stahl USA wascoated onto a PET substrate and the solvent was flashed off. The driedpermuthane is then placed into a mixture of an approximate mixture ofDichloromethane and Methanol (93:7) at an approximate ratio of 12.5:87.5for an approximately 2 weeks to allow the permuthane to fully dissolve.The fully dissolved permuthane is placed into a 9 gallon pressurizedvessel and an air driven lightening mixer with an impeller is used tomake a homogeneous solution. To this permuthane mixture, GOS 3010-03from Nichia Corp. was added together at a ratio of 77:23(GOS:permuthane) and allowed to mix using the air driven lighteningmixer with an impeller. GOS is a gadolinium oxy sulfide of the formulaGd₂O₂S:Tb. The resulting slurry is coated on a 10 mil polyester support,using the slot coater, and transported through a series of air dryerswith a temperature range between about 35° C. to about 46° C. forapproximately 8 minutes.

Scintillator coatings for the following examples were prepared using alab-scale slot coater. The barrier layer materials (CDA, EVA, BUTVAR)polyvinyl acetate (PVAc), polyvinyl alcohol (PVOH) and gelatin wereapplied using the same slot coating techniques, but over the driedscintillator layer. The antistatic layers were coated using a lab scalespin coater (Laurell Spin Coater Model WS-650).

The coating solutions utilized in the generation of the inventive andcomparative examples were solvent-based solutions, therefore, drying wasprimarily performed by thermal and/or evaporative techniques. In thecase of drying or curing, the temperature was above room temperature,and an inert gas environment was utilized if necessary through use of aninert gas oven. In addition, the air flow around the coated layer waslaminar.

Solution ingredients were mixed in a controlled manner, and agitatedusing standard mixing methods (e.g., prop mixing, magnetic bar,sonicator, or roller mill) for a specified amount of time. Then, thestability of the dispersion observed visually, using a light sourcebehind a clear container and observing if there is the noticeablepresence of agglomerations or settling of materials to the bottom of thecontainer.

DRZ+ Screen (Representative of Scintillator Screen Surface with a PETOvercoat that Serves as a Barrier Layer for Subsequent Coatings):

Mitsubishi provides three levels of scintillator screens that utilizeprompt emitting phosphors, DRZ-Std, DRZ-Plus and DRZ-High. All threescreens have a common product structure; a supporting layer, a phosphorlayer and a protective layer. The supporting layer is a “plastic base”that can range from 188 μm to 250 μm; the DRZ+ scintillator has a 250 μmbase thickness. The phosphor layer is comprised of a proprietaryphosphor and binder formulation that ranges in thickness from 140 μm to310 μm; the DRZ+ scintillator has a 208 μm phosphor layer thickness. Theprotective layer is a polyethylene (PET) sheet that covers the phosphorlayer and ranges in thickness from 6 μm to 9 μm.

Optical Measurements

Visual transmittance was measured using a Perkin Elmer Lambda 3B UV/VisSpectrophotometer. This system was used to measure both dispersions andcoated samples.

Surface Resistivity

The measurement of surface resistivity of the coated samples wasconducted in accordance to American Society for Testing and Materials(ASTM) standard D257-07. Resistivity measurement equipment from ProstatCorporation was utilized for all testing. The resistivity measurementkit included the PRS-801 Resistivity meter, PRF-911 Concentric Ring Set(which included the required test leads and connectors), PTB-920 DualTest Bed and the PRS-801-W 5 lb. weight. The instrument was calibratedbefore each use using a shunt that was provided by the manufacturer.

Charge Decay Measurement

The measurement of charge decay was conducted in accordance with thegeneral practices outlined in FTM Standard 101C, method 4046.1.Measurement equipment from Prostat Corporation was utilized for alltesting. The equipment utilized included: the PFM-711A ElectrostaticField Meter, the CPM-720A Charge Plate Assembly, Conductive Probes, thePGA-710 Autoanalysis System, the PCS-730 Charging Source and associatedconnecting cables. All instruments were calibrated at the factory. ThePFM-711A Field meter has a zero balance, this was adjusted and verifiedto be zero before each test.

Clean Room Wiping Protocol

A clean room towel is wetted with isopropyl alcohol (IPA). The towel wasthen used to wipe the antistatic layer coated scintillator screen. Aconstant and repeatable wiping normal force is generated by wrapping thetowel around a 1 kilogram cylindrical weight, that is about 38 mmdiameter. The weight/towel is placed on an 83 mm wide scintillator andis dragged across the surface of the screen. This process is repeated 3times—each time using a different portion of the towel. The ability towithstand abrasion from clean room wiping is demonstrated by the Wipetest. The scintillator screen must be abrasion resistant to maintain ESDprotection.

The experiments described below show the need for a non-moistureabsorbing and non-water soluble barrier layer over a scintillator layerhaving a prompt emitting phosphor dispersed in a polymeric binder.Without the barrier layer, the antistatic layer does not provide theproper level of conductivity due to the porous nature of thescintillator layer 14. The antistatic material has a propensity todiffuse into the scintillator layer 14, distorting the conductivitynetwork, and thus increasing the surface resistivity. In order to reducethe surface resistivity under these circumstances, it is necessary toeither saturate the concentration of the antistatic material thatdiffuses into the scintillator layer 14, which would require asignificant increase in the concentration while reducing transparency ofthe scintillator layer 14, or introduce a barrier layer 18 between thescintillator layer 14 and the antistatic layer 19. The barrier layer 18prevents diffusion of the antistatic material into the scintillatorlayer 14, while maintaining the transparency requirements.

The active ingredient in the antistatic layer formulation of an aqueousdispersion of Clevios Pedot (PEDOT/PSS) available from Heraeus Company.The antistatic species in PEDOT/PSS ispoly(3,4-ethylenedixythiophene)-poly(styrene sulfonate). The addenda ofthe antistatic formulations are outlined in the Table 1 below.

TABLE 1 Ingredient Wt % 1X Binder Formulation Water 79.96 Clevios PH1000 7.69 AQ55D 4.95 Dynol 2.4 Ethylene Glycol 5 2X Binder FormulationWater 74.25 Clevios PH 1000 14.29 AQ55D 4.60 Dynol 2.23 Ethylene Glycol4.63 4X Binder Formulation Water 64.97 Clevios PH 1000 25 AQ55D 4.02Dynol 1.95 Ethylene Glycol 4.06 8X Binder Formulation Water 51.97Clevios PH 1000 40 AQ55D 3.22 Dynol 1.56 Ethylene Glycol 3.25The Eastman AQ55 addenda was dispersed in distilled water, heated to 70°C. and stirred for a minimum of 2 hours until it was completelydissolved and homogeneous. Once the AQ55 solution is complete, theantistatic layer formulation is measured in the ratios provided in theabove table. Dynol 604 from Air Products and Chemicals, Allentown, Pa.is used as a coating aid. Ethylene Glycol is added to the mixture to actas a conductivity enhancer. The solution is mixed by any number oftechniques including the use of a roller mill, and spin coated.

Comparative Example 1

The antistatic formulations shown described above were coated on the CSHScintillator Screen described above. The surface resistivity and percenttransmittance in the 400-600 nm range are shown in Table 2 below.

Comparative Example 2

A gelatin coating was coated on the CSH Scintillator Screen. A gelatinovercoat layer was prepared as follows. 144.5 grams of Gel 34, a limeprocessed ossein was dissolved into 1760.5 grams of distilled water for120 minutes. This mixture was heated up to 50° C. and held for 20minutes. After 20 minutes, the temperature is lowered to 40° C. and aprop mixer was inserted into the container. The mixer was turned on for10 minutes and a surfactant package was added to the mixture. Thismixture was slot die coated on top of the scintillator screen at a wetthickness of 0.005″. After the overcoat solution was applied, it wasdried in place with a platen temperature of 50° C. for 20 minutes.

The antistatic formulations shown in Table 1 were coated on the CSHScintillator Screen with the gelatin overcoat. The surface resistivityand percent transmittance in the 400-600 nm range are shown in Table 2below.

Comparative Example 3

A polyvinyl alcohol coating was coated on the CSH Scintillator Screen asa barrier layer. 20 g of Sigma Aldrich Poly(vinyl alcohol, MW89,000-98,000 was weighed out and mixed with 360 g of Distilled Waterand mixed using a prop mixer for 2 hours at 88° C. This mixture was slotdie coated on top of the scintillator screen at a wet thickness of0.005″. After the overcoat solution was applied, it was dried in placewith the platen temperature of 40° C. for 20 minutes. The antistaticformulations shown described in Table 1 were coated on the CSH screenwith the polyvinyl alcohol overcoat. The surface resistivity and percenttransmittance in the 400-600 nm range are shown in Table 2 below.

Inventive Example 1

The antistatic formulations shown and identified in Table 1 above werecoated on the DRZ+ screen. The surface resistivity and percenttransmittance in the 400-600 nm range are shown in Table 2 below.

Inventive Example 2

An ethylene vinyl acetate (EVA) coating was coated on the CSHScintillator Screen as a barrier layer. 5 grams of DuPont™ Elvax 150 isan ethylene vinyl acetate copolymer resin was weighed out and mixed with95 grams of Toluene and roller milled for 24 hours. This mixture wasslot die coated on top of the scintillator screen described at a wetthickness of 0.005″. After the solution was applied, it was dried inplace with the platen temperature of 40° C. for 20 minutes. Theantistatic formulations shown in Table 1 were coated on the CSHScintillator Screen with the EVA overcoat. The surface resistivity andpercent transmittance in the 400-600 nm range are shown in Table 2below.

Inventive Example 3

A cellulose diacetate (CDA) coating was coated on the CSH ScintillatorScreen as a barrier layer. 2 grams of the Eastman™ Cellulose Acetate(CA-398-3) was weighed out and mixed with 98 grams of NitroMethane androller milled for 2 hours. This mixture was slot die coated on top ofthe CSH Scintillator Screen at a wet thickness of 0.005″. After theovercoat solution was applied, it was dried with the platen temperatureof 40° C. for 20 minutes. The antistatic formulations shown in Table 1were coated on the CSH Scintillator Screen with the EVA overcoat. Thesurface resistivity and percent transmittance in the 400-600 nm rangeare shown in Table 2 below.

Inventive Example 4

A polyvinyl butyraldehyde (Butvar) coating was coated on the CSHScintillator Screen as a barrier layer. 5 grams of Butvar® 76 (Polyvinylbutyral resin) was weighed out and mixed with 95 grams of Acetone androller milled for 24 hours. This mixture was slot die coated on top ofthe CSH Scintillator Screen at a wet thickness of 0.005″. After theovercoat solution was applied, it was dried with the platen temperatureof 12° C. for 20 minutes. The antistatic formulations shown in Table 1were coated on the CSH Scintillator Screen with the EVA overcoat. Thesurface resistivity and percent transmittance in the 400-600 nm rangeare shown in Table 2 below.

TABLE 2 Minimum transmittance in the 400-800 nm range Coating MeetsMinimum Antistat (measured by coating Resistivity Specification <1.0 E5Coating the antistat formulation Surface Resistivity Ω/□ & MinimumTransmission Sample Coating surface Formulation on a glass substrate)(Ω/□) Requirement Comparative a film of GOS in permuthane 1X 95%1.36E+06 No - Fail Resistivity Example 1 2X 94% 3.84E+06 No - FailResistivity 4X 92% 3.50E+04 No - Fail Transmission 8X 88% 1.05E+04 No -Fail Transmission Comparative a thin film of gelatin coated 1X 95%2.00E+11 No - Fail Resistivity Example 2 over a film of GOS in 2X 94%9.55E+09 No - Fail Resistivity permuthane 4X 92% 2.20E+09 No - FailResistivity & transmission 8X 88% 3.55E+08 No - Fail Resistivity &transmission Comparative a thin film of polyvinyl alcohol 1X 95%8.83E+08 No - Fail Resistivity Example 2 coated over a film of GOS in 2X94% 4.76E+08 No - Fail Resistivity permuthane 4X 92% 1.47E+08 No - FailResistivity & transmission 8X 88% 1.87E+06 No - Fail Resistivity &transmission Inventive a thin film of PET coated over 1X 95% 5.55E+07No - Fail Resistivity Example 1 a film of GOS in permuthane 2X 94%2.05E+04 Yes layer 4X 92% 2.45E+04 No - Fail Transmission 8X 88%9.00E+03 No - Fail Transmission Inventive a thin film of CDA coated 1X95% 7.35E+07 No - Fail Resistivity Example 2 over a film of GOS in 2X94% 1.60E+04 Yes permuthane layer 4X 92% 195E+04 No - Fail Transmission8X 88% 8.80E+03 No - Fail Transmission Inventive a thin film of EVAcoated over 1X 95% 2.35E+06 No - Fail Resistivity Example 3 a film ofGOS in permuthane 2X 94% 5.85E+04 Yes layer 4X 92% 1.50E+04 No - FailTransmission 8X 88% 3.90E+03 No - Fail Transmission Inventive a thinfilm of Bulvar coated 1X 95% 5.58E+07 No - Fail Resistivity Example 4over a film of GOS in 2X 94% 6.70E+04 Yes permuthane layer 4X 92%2.20E+04 No - Fail Transmission 8X 88% 8.50E+03 No - Fail Transmission

In the results shown in Table 2, the goal was to find an antistaticlayer that provided excellent surface resistivity and transmission. Thegoal was to have surface resistivity under 10⁵ ohms per square. Previoustests with poly (3,4-ethylene dioxythiophene) as an antistatic agentshowed surface resistivities that were not acceptable. Zelec®electroconductive powders were tested and provided acceptable surfaceresistivity, however, the transmission in the 400 nm to 600 nmwavelength was unacceptable. As the scintillator screen 12 is mated witha FPD 20, there can be an insulating effect caused by the adhesivelayer. In order to overcome this effect it is necessary in the testsabove to be well under 10¹⁰ ohms per square so the DR panel will haveprotection from ESD events. A highly conductive antistatic agent allowsfor thinner coating layers.

Other embodiments of the present teachings will be apparent to thoseskilled in the art from consideration of the specification and practiceof the present teachings disclosed herein. In addition, while aparticular feature of the invention can have been disclosed with respectto only one of several implementations, such feature can be combinedwith one or more other features of the other implementations as can bedesired and advantageous for any given or particular function. The term“about” indicates that the value listed can be somewhat altered, as longas the alteration does not result in nonconformance of the process orstructure to the illustrated embodiment. Further, “exemplary” indicatesthe description is used as an example, rather than implying that it isan ideal. It is intended that the specification and examples beconsidered as exemplary only, with the true scope and spirit of thepresent teachings being indicated by the following claims.

What is claimed is:
 1. A scintillator screen comprising: a supportinglayer; a phosphor dispersed in a polymeric binder disposed on thesupporting layer, a barrier layer disposed on the polymeric bindercomprising a non-moisture absorbing polymer selected from the groupconsisting of polyethylene terephthalate, cellulose diacetate, ethylenevinyl acetate and polyvinyl butyraldehyde, wherein the barrier layer hasa thickness of less than 1 micron; and an antistatic layer disposed onthe barrier layer, the antistatic layer comprising apoly(3,4-ethylenedixythiophene)-poly(styrene sulfonate) dispersed in apolymer selected from the group consisting of a polyester and apolyurethane wherein the antistatic layer has a transparency of greaterthan 95 percent at a wavelength of from about 400 nm to 600 nm.
 2. Thescintillator screen of claim 1, wherein the barrier layer has atransparency of greater than 94 percent at a wavelength of from about400 nm to 600 nm.
 3. The scintillator screen of claim 1, comprising asurface resistivity less than 10¹⁰ ohms/square.
 4. The scintillatorscreen of claim 1, wherein the phosphor is selected from the groupconsisting of Gd₂O₂S:Tb, Gd₂O₂S:Eu, Gd₂O₃:Eu, La₂O₂S:Tb, La₂O₂S,Y₂O₂S:Tb, CsI:Tl, CsI:Na, CsBr:Tl, NaI:Tl, CaWO₄, CaWO₄:Tb, BaFBr:Eu,BaFCl:Eu, BaSO₄:Eu, BaSrSO₄, BaPbSO₄, BaAl₁₂O₁₉:Mn, BaMgAl₁₀O₁₇:Eu,Zn₂SiO₄:Mn, (Zn,Cd)S:Ag, LaOBr, LaOBr:Tm, Lu₂O₂S:Eu, Lu₂O₂S:Tb, LuTaO₄,HfO₂:Ti, HfGeO₄:Ti, YTaO₄, YTaO₄:Gd, YTaO₄:Nb, Y₂O₃:Eu, YBO₃:Eu,YBO₃:Tb, and (Y,Gd)BO₃:Eu.
 5. The scintillator screen of claim 1,wherein the polymeric binder is selected from the group consisting of:sodium o-sulfobenzaldehyde acetal of poly(vinyl alcohol);chloro-sulfonated poly(ethylene); bisphenol poly(carbonates); copolymersof bisphenol carbonates and poly(alkylene oxides); aqueous ethanolsoluble nylons; poly(alkyl acrylates and methacrylates): copolymers ofpoly(alkyl acrylates and methacrylates); poly(vinyl butyral) andpoly(urethane) elastomers.
 6. The scintillator screen of claim 1,wherein the phosphor comprises particles having a size of from about 0.5μm to about 40 μm, wherein a phosphor to polymeric binder ratio is fromabout 7:1 to about 35:1.
 7. The scintillator screen of claim 1, whereinthe antistatic layer has thickness of from about 0.1 μm to about 0.3 μm.8. A digital radiography panel comprising: a scintillator screen havinga supporting layer; a phosphor dispersed in a polymeric binder disposedon the supporting layer, a barrier layer disposed on the polymericbinder comprising a non-moisture absorbing polymer selected from thegroup consisting of polyethylene terephthalate, cellulose diacetate,ethylene vinyl acetate and polyvinyl butyraldehyde; an antistatic layerdisposed on the barrier layer, the antistatic layer comprising apoly(3,4-ethylenedixythiophene)-poly(styrene sulfonate) dispersed in apolymer selected from the group consisting of a polyester and apolyurethane wherein the antistatic layer has a transparency of greaterthan 95 percent at a wavelength of from about 400 nm to 600 nm; and aflat panel detector disposed on the antistatic layer.
 9. The digitalradiograph panel of claim 8, wherein the barrier layer has atransparency of greater than 95 percent at a wavelength of from about400 nm to 600 nm.
 10. The digital radiograph panel of claim 8, whereinthe antistatic layer has a surface resistivity less than 10⁵ohms/square.
 11. The digital radiograph panel of claim 8, wherein thephosphor is selected from the group consisting of Gd₂O₂S:Tb, Gd₂O₂S:Eu,Gd₂O₃:Eu, La₂O₂S:Tb, La₂O₂S, Y₂O₂S:Tb, CsI:Tl, CsI:Na, CsBr:Tl, NaI:Tl,CaWO₄, CaWO₄:Tb, BaFBr:Eu, BaFCl:Eu, BaSO₄:Eu, BaSrSO₄, BaPbSO₄,BaAl₁₂O₁₉:Mn, BaMgAl₁₀O₁₇:Eu, Zn₂SiO₄:Mn, (Zn,Cd)S:Ag, LaOBr, LaOBr:Tm,Lu₂O₂S:Eu, Lu₂O₂S:Tb, LuTaO₄, HfO₂:Ti, HfGeO₄:Ti, YTaO₄, YTaO₄:Gd,YTaO₄:Nb, Y₂O₃:Eu, YBO₃:Eu, YBO₃:Tb, and (Y,Gd)BO₃:Eu, and wherein thepolymeric binder is selected from the group consisting of: sodiumo-sulfobenzaldehyde acetal of poly(vinyl alcohol); chloro-sulfonatedpoly(ethylene); bisphenol poly(carbonates); copolymers of bisphenolcarbonates and poly(alkylene oxides); aqueous ethanol soluble nylons;poly(alkyl acrylates and methacrylates): copolymers of poly(alkylacrylates and methacrylates); poly(vinyl butyral) and poly(urethane)elastomers.
 12. The digital radiograph panel of claim 8, furthercomprising and adhesive layer disposed between the antistatic layer andthe flat panel detector.
 13. The digital radiograph panel of claim 8,wherein the barrier layer has a thickness of less than 1 micron, whereinthe phosphor comprises particles having a size of from about 0.5 μm toabout 40 μm, wherein a phosphor to polymeric binder ratio is from about7:1 to about 35:1, and wherein the antistatic layer has thickness offrom about 0.1 μm to about 0.3 μm.
 14. A scintillator screen comprising:a supporting layer; a phosphor dispersed in a polymeric binder disposedon the supporting layer, a barrier layer disposed on the polymericbinder comprising a non-moisture absorbing polymer; and an antistaticlayer disposed on the barrier layer, the antistatic layer comprising apoly(3,4-ethylenedixythiophene)-poly(styrene sulfonate) dispersed in apolymer selected from the group consisting of a polyester and apolyurethane wherein the antistatic layer has a transparency of greaterthan 95 percent at a wavelength of from about 400 nm to 600 nm andwherein the antistatic layer has thickness of from about 0.1 μm to about0.3 μm.
 15. The scintillator screen of claim 14, wherein the antistaticlayer has a surface resistivity less than 10¹⁰ ohms/square.